In the prior art string of bulbs (for decoration purpose), the structure thereof can not but use the series connection method between the bulb seats (or bulbs). Consequently, the reliability of the string of bulbs is rather low; for example, one bulb being out of order will jeopardize the power supply of the whole string of bulbs. Recently, a shunt bulb has been developed, but the length of the string is still limited. Further, the conventional string of bulbs presents a salient or conspicuous appearance, which would adversely affect the decorative effect. The small bulb seat for the conventional string of bulbs is difficult to manufacture (requiring higher precision), and it is rather difficult to develop the smallest string of bulbs to be used as a decoration for smaller items. Such small bulb strings also require longer manufacturing time thereby increasing the basic cost, which renders it of less industrial value. Therefore, the target of industry in that field has been set in the areas of how to make every effort to shorten the manufacturing time, to lower the basic cost, and to increase reliability of the string of bulbs.